Since electromagnetic wave used for a satellite broadcast is in a microwave frequency band, when the level of reception is lowered by rainfall, snow on an antenna, etc., C/N (carrier/noise) of a received signal is also lowered. As C/N is lowered, S/N (signal/noise) of an image signal after FM demodulation is lowered. When C/N is further lowered and it falls below a threshold value, a particular kind of impulse noises are produced.
FIG. 2 (b) shows waveform of this kind of impulse noises produced in an image signal. When C/N is lowered, impulse noises B are produced, as indicated in FIG. 2 (b), on an image signal A, whose brightness varies stepwise, as indicated in FIG. 2 (a). When these impulse noises are produced, the brightness is raised or lowered significantly over several hundreds of nanoseconds. When they are observed on a monitor screen, they look like white or black killifish, which impairs remarkably the image quality. In order to eliminate such impulse noises, there is known a noise eliminating circuit as indicated in FIG. 7 disclosed e.g. in Technical Report of Juridical Foundation Television Society, Vol. 14, No. 42, pp. 16-18 , August 1990. In FIG. 7, reference numeral 51 is a digital noise filter; 52 is a differential circuit; 53 is a comparing circuit; and 54 is an error detection preventing circuit. At first, a demodulated signal is A-D converted and color subcarrier and digital aural subcarrier are removed by means of the digital notch filter 51. Then the high frequency component of the image signal is taken out by means of the differential circuit 52. The signal is compared with a threshold in the level comparing circuit 53 and pixels exceeding the threshold value are judged to be suspected to be impulse noises. Further, in order to prevent erroneous detection, only when the none of two adjacent pixels on the upper and lower sides of each of the pixels is suspected to be an impulse noise, it is judged finally to be an impulse noise.
Impulse noises are eliminated by detecting the impulse noises from a demodulated signal of the image signal by using this noise eliminating circuit and by replacing the detected pixels including the impulse noises by pixels including no impulse noises, which succeed or precede them by one field or one scanning line. Concretely speaking, when the pixels to be replaced are selected, they should be replaced by pixels located obliquely on the upper or lower side succeeding or preceding by one field or one scanning line, as indicated in FIGS. 3(a) and 3(b), so that the phase of the color subcarrier multiplexed in the frequency in the image signal is continuous after the replacement of the pixels.
Such a prior art impulse noise detecting circuit, which detected noises on the basis of horizontal and vertical correlation in one field, had a drawback that impulse noises were apt to be erroneously detected.
Further, in the prior art circuit, there was a problem that since the composite image signal is interpolated, as it is, it is necessary to adjust the phase of the color subcarrier and that for this reason the interpolation should be effected as indicated in FIG. 3, and thus the precision of the interpolation is low, because the interpolation is effected by using pixels obliquely adjacent.
As other prior art techniques JP-A-Sho 63-110888, 110889 and 110890 can be cited. However these techniques have a drawback that detection precision is low, because they deal with an NTSC signal itself and noise elimination is effected before the YC separation.